Extractive distillation process



Patented Oct. 2, 1951 2,570,066 EXTRACTIVE DISTILLATION PROCESS Morris' R. Morrow and Jack W. Smalling, Baytown, Tex., assignors, by mesne assignments, to

Standard Oil Development Company,

Elizabeth, N. J., a corporation oi Delaware Application August 26; 1948, Serial No. 46,196 1 claim. (Cl. 2oz- 395) The present invention is directed to a method of segregating pure hydrocarbons from hydrocarbon mixtures. More particularly, the invention has to do with extractive distillation of hydrocarbon mixtures to obtain a substantially pure hydrocarbon. In its more particularaspects, the

invention is concerned with the extractive distillation of a hydrocarbon mixture in the presence of an aromatic hydrocarbon solvent.

In petroleum refining operations, it is often desirable to produce substantially pure paramn hydrocarbons from hydrocarbon mixtures thereof. vThe pure parallins obtained from petroleum refining operations haveV included iso-octane, normal heptane, and higher boiling paramnic mixtures -for use as reference fuels, solvents, specialties, duce corresponding aromatic hydrocarbonsl or cycloparaillnic hydrocarbon compounds for use as solvents, specialties, `motor andv aviation gasoline components and as chemical raw materials. For example, cyclohexane is an important starting point in the production of adipic acid which is used in synthetic fiber production. The conventional practice in separating aromatic hydrocarbons from other hydrocarbons has been to extract the hydrocarbon mixtures containing the aromatics with selective solvents such assulfur dioxide. phenol, aniline. furfural, nitrobenzene, and cresolic mixtures. Sepa-ration of cycloparaillns from acyclic parailins has not been developed to any great extent because the separation betweenA acyelicl paraiiinic components and cycloparalllnic components is very dimcult and usually requires fractional distillation in very highly efficient distillation towers in combination rwith the socalled extractive distillation or vapor-liquid extraction where the boiling points between the cycloparaillns and the paraillns are not sutllciently great. Extractive distillation has been used, for example, in separating olefinic hydrocarbons and the like. It is also desirable to profrom parafllns, such as the separation o1' 'butylenes from the butanes.

vOne great objection'to extractive distillation has been the high initial cost of installing equipment and the cost of operating it. The solvents usually employed in extractiva distillation are quite expensive and are not available in the reilnery as a by-product and are often quite poisonous, requiring elaborate precautions operating personnel. the foregoing discussion that there is a great need for an extractive distillation process employing a solvent which is readily available in most oil refining operations.

It is, therefore, the main object of the present invention to provide an extractiva distillation process employing' a hydrocarbon solvent.

'Another objectof the present invention ls to provide a process for separation of one hydrocar-l to protect Thus, it will be seen from l solvent boiling inthe range between 365 bon component from another hydrocarbon component in a mixture thereof.

Another object of the present invention is to separate by extractive distillation employing a hydrocarbon solvent mixtures of cycloparailins and paraillm and other hydrocarbon mixtures into their component parts.

It has now been found that separation of various hydrocarbon mixtures, such as mixtures of cycloparaiflns and paraiilns, paraiilns and aromatics, oleilns and paraiins, paraillns and naphthenes, and naphthenes and aromatics, may be accomplished by an extractive distillation process employing as a solvent a mono-cyclic aromatic hydrocarbon fraction boiling in the range between 365 and 750 F.

The present invention may be described briefly as involving the heating of a narrow boiling hydrocarbon fraction in the gasoline boiling range such as, for example, a fraction boiling in the range between 145 and 160 mal hexane and methylcyclopentane, to a temperature of approximately the boiling point of the heaviest component of the mixture, following which the heated hydrocarbon fraction is contacted with a monocyclic aromatic hydrocarbon and 750 F. under conditions that the mixture of parafflnlc and cycloparailinic constituents is substantially in thevapor phase and the aromatic solvent is in the liquid phase so that a separation is obtained between the methylcyclopentane and the normal hexane. The normal hexane in this particular instance will be obtained as the railinate while the methylcyclopentane will be dissolved in the solvent and removed therewith to a solvent recovery tower where it may be separated by virtue of the dilerence in boiling points and recovered, with the solvent being recycled and reused in the process.

In conducting the extractive distillation in accordance with the present invention, it will' be desirable to employ feed stocks, of the'various mixtures mentioned before, having a boiling range no greater than about 50 F. to obtain the greatest benefit of the new extractive distillation process. While a 50 boiling range fraction may be employed in the practice oi the present invention, it will usually be preferred to employ a feed having a boiling range no greater than 25"A F. which -will usually include only two component mixtures. Fractions having boiling ranges of 50 y spread will include more thany two components and the practice of the present invention may be used to obtain one component in a purified form from say twov other components. Ordinarily. such fractions fed to the process of the present invention will contain two components and the feed stock should be so prepared that it will be composed of two types of hydrocarbons. For ex- F. containing norl and contacted with the monocyclic aromatic 3 f ample, a feed stock may include a paraflinic component and a cycloparaiiinic component. The aromatic hydrocarbon solvent employed in the present invention will contain substantially mono-cyclic aromatic hydrocarbons to the exclusion of polycyclic hydrocarbons and may boil in the range from about 385 to 750 F. The solventl may be obtained from an aromatization process or the so-called hydroforming process which is well known in the industry. The fraction boiling between 365 and 150 F. containing substantially monocyclic aromatic hydrocarbons having or more carbon atoms gives good results in the practice of the present invention.

The monocyclic aromatic hydrocarbons having lo carbon atoms are particularly desirable solvents in the practice of the present invention. Such monocyclic aromatic hydrocarbons will include the tetramethylbenzenes such as 1,2,4,5 tetramethylbenzene, 1,2,3,5tetramethylbenzene, and 1.2.3.4-tetramethylbenzene, 1,2-dimethyl-3- ethylbenzene, 1,2-dimethyl--i-ethylbenzene, and the like. Especially beneficial solvents for use in the present invention are the tetramethylbenzenes including durene, iscdurene, prehnitene, and mixtures thereof.

As mentioned before, the temperatures em-l ployed in the practice of the present invention will depend upon the boiling point of the feed stock. 4As a general rule, it may be stated that the feed stock should be heated to the boiling point of the highest boiling component therein vent at a temperature in the range between the boiling point of the heaviest component in the feed stock and about 50 higher. Stating this another way, the extractive distillation may be performed in a suitable distillation tower with a temperature gradient being maintained across the tower. The temperature at the top of the tower will be about the boiling point of the highest boiling component in the feed mixture while the temperature at the bottom of the -tower will be at least about 50 greater.

'I'he ratios of solvent to feed stock may range from about 1 to 1 to about 20 to 1. Ordinarily, it will be desirable to use about five volumes of tion of the tower above the feed inlet being the extraction stage and that below the feed inlet beingthe stripping stage. The portion of tower above the solvent inlet may also be designated as a product purifying or rectifying stage. 'Ihe feed. in a vaporous form, flows upwardly in tower I4 and countercurrently contacts aromatic solvent boiling in the range between 365 and '150 F. comprising substantially only monocyclic aromatic hydrocarbons, which is introduced by line I3. As the solvent and hydrocarbon feed contact each other a separation is effected between' paraflinic constituents thereof and the cycloparaillnic or naphthenic constituents, the latter being dissolved in the solvent whereas the paraffinic constituents are separated as a rainate. The raiilnate comprising substantially the parafflnic hydrocarbon, normal hexane, discharges from extractive distillation tower I4 by line I5. is cooled and condensed in condenser I6, and is discharged into reflux accumulator drum I1. The

sol-

solvent to one volume of feed stock. The actual L amount selected will depend. of course, on the composition of the feed mixture being subjected to the extractive distillation and the product purity desired.

The invention will now be further illustrated by reference to the drawing in which the single figure presentsa flow diagram of one method of practicing it.

Referring now to the drawing, numeral II designates a charge line through which a feed mixture, for example a mixture of methylcyclovpentane and normal hexane boiling in the range between 145 and 160 F.; is introduced into the system from a source not shown. The feed mixture discharges by line Il into a heat exchange 'device I2 where it is passed in heat exchange with the solvent flowing through line I3 as will be described further. The heat exchange causes the feed stock to be heated to a temperature of about 160 F. and'it is discharged at this temperature into an extractive distillation tower I4 which is provided with suitable internal baffling equipment such as bell cap trays or other packing material to insure intimate contact between vapors and liquids. The feedstock is introduced at about a midpoint in tower I4 so that the tower is thereby divided into what may be called an extraction stage and a stripping stage, the poraccumulated hexane in drum I1 is withdrawn by line I8 and pump I9 and divided into two streams, one stream being injected back into the top of tower I4 by line 20 to provide reflux and the other portion withdrawn by line 2| as prodtower I4 and returned to the base of tower I4 by line 28. The portion of the extract discharged by line 22 into solvent recovery tower 24 issubjected to distillation to recover overhead by line 29 a fraction consisting substantially of the naphthenic components of the feed mixture. This fraction is cooled and condensed in condenser 30 and accumulated in reflux accumulator 3l from whence it is discharged by line 32 and pump 33 for division into two streams, one of which returns to solvent recovery tower 24 by line 34 to provide reflux therein and the other is discharged by line 35 as naphthenic solvent product. Should the naphthenic product contain substantial quantities of low boiling aromatics, a second extractive-distillation step may be used to obtain pure naphthenes and pure aromatic products.

' rIfhe aromatic solvent is withdrawn from near the base of tower 24 by line 36 and is pumped by pump 31 to line I3 where it is returned as has been described to extractive distillation tower I4. Heat is provided for recovery of the naphthenic components in tower 24 by withdrawing from the bottom thereof by line 38 heavy fractions which are pumped by pump 39 and line 40 into a reboilerl or heater 4I where heat is provided on passage through-coil 42 to cause distillation .in tower 24. The heated heavy fractions are returned lto the base of tower 24 by line 43. Y To prevent buildup of heavy fractions in the system, it may be necessary from time to time to discharge a solvent slop fraction from the system and this may be accomplished conveniently' by opening valve 44 in line 40.

To compensate for heavy fractions which may be withdrawn from the system, it may be necessary from time to time to add additional solventto the system. 'I'his additional solvent may be introduced from raw solvent storage tank 45 by way of line 46 and pump 41 which connects into line i3. Under some conditions, it may be desirable to discharge some of the recirculated solvent into the solvent storage tank I5 and this virtue of withdrawing a slop stream of solvent' since this slop stream will include only the highest boiling components. The recirculated solvent will become more and more purified as it circulates in the system. It may be desirable, therefore, in some instances to withdraw a. portion of the recirculated solvent by line 50 controlled by valve 5| for use as maybe desired.

It will be seen from the foregoing description that a simple and eifective extractive distillation process has been provided to obtain separation between hydrocarbon components of a mixture having narrow boiling ranges.

In order to illustrate the invention further, references may be made to the following tabulation in which the relative volatility of a mixture of a parailln and a typical naphthene, as represented by normal heptane and methylcyclohexane, in the presence of conventional solvents is compared with their relative volatility in the presence of a solvent having a boiling point between 365 and 750 F. and containing substantially only monocyclic aromatic hydrocarbons, in accordance with the present invention.

Relative Volatility in Presence of Solvent (Normal Hep tane/Methylcyclohexane) Weight T, Hydro- `rbon in Solution Solvent Aniline Do Phenol Monocyclic Aromatic Hydrocarbons boiling between 365-750 F e ses le 'sae The relative' volatilities shown in the foregoing table are calculated in accordance with the following equation, based on Raoults law:

where 71:0 z is the selectivity of adsorption, Y1 is the mole fraction of paralllns in the vapor, X1 is the mole fraction of paraillns in the liquid, Y2 is the mole fraction of naphthene in the vapor, and X2 is the mole fraction of naphthenes extractiva distillation with a polar solvent since storage problems are not complicated by highly dissimilar materials andthe danger from poisons is substantially less when conducting extractive distillations employing a monocyclic aromatic hydrocarbon solvent boiling in the range between 365 and 750 F. than when employing a solvent such as aniline, phenol or furfural. Y Another great advantage of the solvent employed in the present invention is its relatively low cost since it may be available in most refining operations as a by-product stream.

The invention has been described by an example in which the aromatic hydrocarbon solvent boiling in the range between 365 and '750 F. has been employed in one tower to remove one component from two or more components. It will be apparent to the skilled workman that two or more towers may be used to separate a plurality of components from each other. Thus. a. mixture of parafiins, naphthenes, and aromatics may be extractively distilled in accordance with the present invention to separate paraillns from naphthenes and aromatics which are dissolved in the solvent. In the solvent recovery tower the naphthenes and aromatics lower boiling than the aromatic hydrocarbon solvent would be separated by distillation from the latter and the mixture of the two may then be extractively distilled in a second tower to separate the naphthenes from the aromatics. Similarly, a mixture of parainns and naphthenes may be separated from aromatics in one tower and the mixture then subjected again to extractive distillation to separate parafllns from naphthenes. Nu-` merous modifications of the invention will suggest themselves to the skilled workman.

The nature and objects of the present invention having been fully described and illustrated. what we wish to claim as new and useful and to secure by Letters Patent is:

An extractive distillation method for separating naphthenes from paramns which comprises countercurrently contacting a vaporized liquid mixture of a naphthene and a parailln boiling in the gasoline range and having a boiling range no greater than F. in a distillation zone with tetramethylbenzene at a. temperature above the boiling range of the mixture to form a raillnate phase including the paraffin and an extract phase comprising the naphthene and tetramethylbenzene, and separating said phases.

MORRIS R. MORROW. JACK W. SMALLING.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 2,123,642 Wiezevich July 12, 1938 2,168,691 Voorhees Aug. 8, 1939 2,433,286 McKinnis Dec. 23. 194'? 2,434,424 Morris et al. Jan. 13, 1948 2,455,803 Pierotti Dec. 7, .1948

OTHER REFERENCES Griswold et al.: Pure Hydrocarbons from Petroleum, Industrial and Engineering Chemistry, vol. 38, pages -71 and 170-177 (January and February 1946). 

